In the manufacture of the ceramic-bodied arc tube in a high pressure sodium discharge lamp, problems have been encountered in mounting the electrode rod to its mounting support whether that support be a mounting strap or the exhaust tubulation of the lamp. The electrode rod is generally tungsten and it is usually welded to a refractory metal support or alternatively mechanically mounted to the electrode support.
Direct welding of the tungsten rod to the niobium or tantalum electrode support generally provides a poor weld. If insufficient heat is created during the welding process, the tungsten will not bond sufficiently to the refractory metal support. Alternatively, if sufficient heat is generated by the weld, the tungsten rod will become embrittled and, during handling of the lamp may fracture just above the weld point.
Mechanical clamping of the electrode rod to the electrode support apparently does not provide a tight enough grip to prevent loose or wobbly electrode mounts and in some instances variable mounting heights between the electrode and the end cap or end closure of the arc tube body are encountered.
Some success has been accomplished by combining mechanical clamping with welding. Examples of more successful techniques may be found in U.S. Pat. Nos. 3,882,344 and 3,872,341. Even with this combination of mechanical clamping and welding, loose electrodes, poor welds, and embrittled electrode rods have been encountered.
Good electrode mounts can be critical to the performance and longevity of a high pressure sodium discharge lamp and it is therefore important that the interconnection between the electrode rod and its support be both of good quality and consistently reproducible.